1. Field of the Invention
The field relates to an organic light emitting display.
2. Description of the Related Technology
An organic light emitting display has beneficial aspects of being thin, having a wide viewing angle and high speed. The organic light emitting display can control the brightness of each pixel and display an image by controlling the amount of current which flows through an organic light emitting diode (OLED). In the display, once a current corresponding to a data is supplied to an organic light emitting diode, the organic light emitting diode emits light corresponding to the current supplied. The data applied to the organic light emitting diode has a quantized grey scale value within a predetermined range in order to express a grey scale.
When a thin film transistor which has amorphous silicon (a-Si) is used as a driving transistor, it has a weakness in that current driving ability can be relatively low. However, it also has advantages in that the uniformity of the display device is excellent, and it is more suitable for being manufactured in a large size display. The uniformity of the luminance of the display panel can be low because a driving transistor of the respective pixel circuits of the organic light emitting display can have different threshold voltages from one another. Furthermore, one portion of the panel may be brighter than another because IR-drop occurs in a power supply line (VDD) connecting the respective pixel circuits one another. Moreover, in case that the pixel circuit of the organic light emitting display includes many transistors, it is difficult to achieve high resolution of the panel because high integration becomes impossible. In the case of conventional circuits for compensating for the threshold voltage of a driving transistor in the pixel circuit, a path from a control electrode of the driving transistor to a negative power supply is formed, and then a leakage current can flow through the path. Consequently, it can cause an improper emission of the organic light emitting diode.
In addition, in case that RGB data signals are applied to the pixel circuits using a demux, if the emission control signals applied through the emission control line coupled to the pixel circuits are turned off, the RGB data signals can be stored in a storage capacitor of the pixel circuit improperly. When RGB data signals (voltages) are applied continuously by driving the RGB data signals (voltages) to the storage capacitors not yet initialized, accurate RGB data signals (voltages) cannot be stored in the storage capacitors properly.
In the case of a color organic light emitting display, a color display can be accomplished by including the display device with an organic light emitting diode which emits light of three colors of red, green and blue. However, the materials used as an organic light emission layer can be degraded by the heat generated during emission. Because of the degradation, the luminance of the organic light emitting diode can deteriorate. As a result, the life span of the organic light emitting diode can be decreased. Because the degree of the degradation of an organic light emission layer which forms a red, green and blue organic light emission layer differs from one another, the difference of the luminance of the red, green and blue organic light emission layer can become larger as time goes by. Accordingly, the desired color cannot be reproduced accurately because transition of the color data occurs as the white balance is changed compared with the initial value. Because each emission layer corresponding to red, green and blue color has a different life span from one another, it is difficult to maintain the white balance when the emission layer is driven for a long time.